As an example, he said that a local florist with a Valentine's Day offer could share email addresses or data on who had visited its website which Twitter could then match to its accounts and pitch the ad accordingly.

"This is how most other companies handle this practice, and we don't give advertisers any additional user information," noted Weil.

A limited trial in the US could be rolled out more widely if it is successful with users and advertisers.

Simon Mansell, chief executive of TBG, a social media marketing agency, told the Financial Times that Twitter benefited from being second to market with retargeting. "Facebook has already worked through privacy and legal concerns, which saves Twitter some time when launching new products," he said.

Weil stressed that users were being offered "simple and meaningful privacy options", where they could uncheck one box in their account settings if they wanted to opt out of the scheme.

This approach was welcomed by Electronic Frontier Foundation, the online rights group.

"It is possible to exist in an ecosystem of tailored advertisements and online tracking while also giving users an easy and meaningful opt-out choice," observed the EEF's Adi Mamdar in a blogpost.

"We think Twitter is setting an important example for the internet," he added, noting that its stance was in "stark contrast to many other advertising and tracking firms."